Bosch has developed a direct injection system that greatly improves fuel economy of internal combustion engines. Link to their brochure.
"High-pressure injector HDEV5 Technical features
System pressure     ≤ 20 MPa...
...
Droplet size SMD (Sauter Mean Diameter)     15 μm"

Now, the droplets have a mean diameter of 15 μm. Small, isn't it? That means one millimeter (0.04") can accommodate 67 such droplets side by side in it. A question arises, how many gasoline molecules does a droplet of 15 μm (15x10^-6m) have?

Now, gasoline is a mixture of various hydrocarbons, and in order to calculate the mean molecular weight of the mixture we should have the composition of that mixture with molecular weight of each fraction considered. Therefore, in order to simplify things, let's assume that we have the perfect gasoline in our tanks, that of Octane C8H18.

As a starting point consider that Octane has a molecular weight of 114g/mol (8x12 + 18). Calculating the volume of a droplet we get  4/3 x Pi x (7.5x10^-6)³  = 1.767x10^-15m³  = 1.767x10^-12litres. Gasoline has a density of 750g/liter. Therefore, the weight of each droplet is 1.767x10^-12x 750 g = 1.325x10^-9g. Now, how many mols is that? By dividing 1.325x10^-9g by 114 g/mol we get 1.162x10^-11  mol.

One mol, according to Amadeo Avogadro, has 6.022x10²³ molecules or atoms in it. That is the famous Avogadro's number. Therefore, a droplet of our Octane gasoline has 1.162x10^-11  mol x 6.022x10²³/mol = 6.997 x 10^-11  molecules in this state of the art Bosch direct injection system. That is close to 7 000 000 000 000 molecules in just one tiny droplet of gasoline.

Now burning of a gasoline droplet surrounded by air consisting of some 20% of oxygen and 79% of nitrogen molecules starts from its surface where it has contact to the oxygen, leaving the interior of the droplet last. The smaller the size of the droplet, the more efficient the combustion.

There is a technique of, instead of injecting gasoline into the combustion chamber, letting in gasoline vapor and air. Now, the vaporized gasoline has no droplets, but the gasoline molecules are all separate surrounded by air. So the "droplet" size is one molecule only. It couldn't be less.

Therefore, combustion of gasoline vapor is much more efficient than that of any system of gasoline injection. That means a huge increase in fuel economy. 

Story of Charles Nelson Poque and his super carburettor

Vapor fuel system

Running on vapor

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There is one drawback. As the combustion of gasoline is almost complete, there remains very little to lubricate the valves. That unburnt gasoline used to take care of that. Therefore, just using vaporised gasoline in your engine will reduce the lifetime of the valves. You have to consider adding something else in the combustion chamber for that missing lubrication.